Electrostatic particle removal and characterization

ABSTRACT

An electrostatic decontamination method and decontamination device (10) is disclosed for decontaminating the surface of a semiconductor substrate. The decontamination device (10) includes particle ionizing device (24) that charges contaminating particles (26) on the surface of semiconductor substrate (16) thereby creating ionized particles. Decontamination device (10) also includes substrate biasing device (12) for creating a charge accumulation layer (14) at the top of semiconductor substrate (16) so that the charge accumulation layer (14) has the same charge sign as the ionized particles. In addition, the invention analytically characterizes particles using contaminating particle isolator (44) which contains a particle ionizing device (24) that charges contaminating particles (26) on the surface of semiconductor substrate (16) thereby creating ionized particles. Contaminating particle isolator (44) includes substrate biasing device (12) operable to create charge accumulation layer (14) at the top of semiconductor substrate (16) so that the charge accumulation layer (14) has the same charge sign as the ionized particles. Contaminating particle isolator (44) also includes particle collector (46) that collects the ionized particles. This permits characterizing the particles to determine their chemical composition.

This is a division, of application Ser. No. 08/166,321, filed Dec. 10,1993, now allowed.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to semiconductor device fabrication,and more particularly, to a method and apparatus for electrostaticallydecontaminating the surface of a semiconductor substrate.

BACKGROUND OF THE INVENTION

The need to remove contaminating particles from semiconductor surfacesduring the fabrication process has grown increasingly important as thesize of semiconductor devices decreases. In an attempt to conquer thisproblem, manufacturers of semiconductors have employed two majortechniques. A first group of manufacturers considers it impossible toremove particles once they attach to a surface, especially as theparticles and level of integration reach submicron sizes. To solve thecontamination problem, this group of manufacturers advocates takingpreemptive actions to preclude contamination in the first place. Asecond group of manufacturers accepts the reality of contamination andadvocates reducing contamination through the use of particle removaltechniques.

Members of both groups currently use methods that suffer from seriousdeficiencies. As engineers continue to develop smaller, submicron-sizeddevices, the significance of submicron-sized contamination particlesincreases. For those in the first group of manufacturers, existingtechnology becomes less efficient as the contaminating particle sizedecreases. First, it becomes more and more difficult to design cleanrooms and equipment that will prevent submicron-sized contaminationparticles from reaching the surface of a substrate. Second, existingtechniques for characterizing contaminating particles are oftenineffective for particles in the submicron range. Existing probes do nothave a fine enough resolution to isolate contaminating particles of thissize. Without an effective technique for characterization, manufacturerscannot identify the source of contamination. As a result, manufacturershave great difficulty eliminating these unknown contamination sources.

Those techniques used by members of the second group of manufacturersalso have their failings. One popular technique employs brush scrubbingwith water or isopropyl alcohol. This technique is ineffective even formicron-sized particles as it is nearly impossible to make a brushcapable of reaching every micron of a surface even when used with aliquid.

Others in the second group of manufacturers advocate the use of highenergy irradiation to remove particles. This controversial technique hasnot been proven effective and also risks damaging the substrate.

Members of the second group of manufacturers most commonly use a liquidsurfactant solution for particle removal. When using this technique, asubstrate is immersed in surfactant and the solution is normallyagitated with a transducer operating at high frequencies. Liquidparticle removal, however, suffers from several disadvantages. Theefficiency of the technique decreases as the size of the contaminatingparticles decreases, making this process of decreasing importance forcurrent and future generations of semiconductor processing. Currenttechniques are, at best, normally only effective for particles of micronsize or larger. Liquid decontamination techniques are normallyineffective at the submicron level because the agitation created in theliquid frequently causes a boundary layer to develop on the surface witha thickness of approximately 0.2 to 0.3 microns. The boundary layer inthe liquid prevents particles of this size, or smaller, from beingaffected by the turbulence created in the liquid. In addition, theliquids used for decontamination can actually cause contamination of thesubstrate. In particular, these liquids can leave both particulate andmetallic contamination on the substrate. Chemicals also tend to beexpensive to purchase and to dispose of.

SUMMARY OF THE INVENTION

Therefore, a need has arisen for a way to decontaminate semiconductorsurfaces that avoids the use of chemicals and effectively removesparticles smaller than one micron. In addition, a need has arisen for atechnique for characterizing submicron-sized particles that allowsaccurate identification of these particles.

In accordance with the present invention, a method and apparatus isprovided that substantially eliminates or reduces disadvantages andproblems associated with prior methods and apparatus for decontaminatingsemiconductor surfaces. In particular, a method for electrostaticallydecontaminating the surface of a semiconductor substrate is providedwherein contaminating particles on the semiconductor substrate areionized to produce ionized particles. Also, the semiconductor substrateis biased to produce a charge accumulation layer at the top of thesubstrate, the layer having the same charge sign as the ionizedparticles. Accordingly, contaminated particles become electrostaticallyseparated from the semiconductor surface.

An important technical advantage of the present invention is that itallows for the removal of particles much smaller than one micron. Theinvention is also a dry process, eliminating the need to purchase anddispose of expensive chemicals. Moreover, the invention eliminates theneed to expose humans to hazardous chemicals in particle decontaminationoperations. The disclosed invention also allows particle removal withoutthe risk of further contamination of the surface or the semiconductormaterial itself. Particles can be removed at a lower cost using thedisclosed invention. The invention also allows easy collection ofsubmicron-sized particles for characterization. Consequently, thedisclosed invention solves many of the problems with currently usedmethods and provides a decontamination method attractive to both groupsof manufacturers.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates one embodiment of an apparatus for electrostaticallydecontaminating a semiconductor surface made in accordance with theteachings of the present invention;

FIG. 2 illustrates the operation of the electrostatic decontaminationapparatus shown in FIG. 1;

FIG. 3 illustrates one embodiment of a electrostatic decontaminationdevice made in accordance with the teachings of the present invention;

FIG. 4 illustrates the operation of the decontamination device shown inFIG. 3;

FIG. 5 shows one embodiment of a contaminating particle isolator made inaccordance with the teachings of the present invention; and

FIG. 6 shows one embodiment of a contaminating particle isolator made inaccordance with the teachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the present invention and its advantages arebest understood by referring to the FIGS. 1-6, like numerals being usedfor like and corresponding parts of the various drawings.

FIG. 1 illustrates one embodiment of a decontamination device, indicatedgenerally at 10, for electrostatically decontaminating the surface of asemiconductor substrate in accordance with the teachings of the presentinvention. Decontamination device 10 includes substrate biasing device12, that creates a charge accumulation layer 14 at the top ofsemiconductor substrate 16. In this embodiment, substrate biasing device12 consists of substrate biasing circuitry 18 that connects to waferchuck 20. Wafer chuck 20 includes insulating layer 22.

Decontamination device 10 further includes one or more particle ionizingdevices 24 that are operable to charge contaminating particles 26 sothat the contaminating particles have the same charge sign as doescharge accumulation layer 14. In the FIG. 1 embodiment, particleionizing devices 24 include a source of deep ultraviolet light orradiation. One particle ionizing device 24 could also be used instead ofmultiple particle ionizing devices. For most decontamination operations,a low energy source of deep ultraviolet light suffices. The position ofparticle ionizing devices 24 relative to the semiconductor substrate 16is unimportant, provided that the deep ultraviolet photons that particleionizing devices 24 give off reach semiconductor substrate 16.

To aid in removing particles, this embodiment also includes gas flowsource 28 that supplies a moving flow of gas over the surface ofsemiconductor substrate 16. Gas flow source 28 supplies a gas that isinert to semiconductor substrate 16. Any gas inert to the substrate canbe used such as electronics grade helium or argon. The gas flow sourcemay also include a filtering mechanism (not shown) capable of removingparticles of the magnitude 0.1 microns or smaller.

This embodiment of decontamination device 10 also includes containmentvessel 30 which prevents further contamination of semiconductorsubstrate 16. Containment vessel 30 includes one or more gas flow inputopenings 32 and gas flow output openings 34. Gas flow input opening 32allows gas supplied from gas flow source 28 to flow into containmentvessel 30 and across the surface of semiconductor substrate 16. The gasthen flows out of containment vessel 30 through gas flow output opening34. Particle collection vessel 36 connects to gas flow output opening 34and collects particulate contaminants. Many suitable methods forcollecting contaminated particles can be used including filtration andelectrostatic collection. Particle collection vessel 36 also connects toused gas containment vessel 38 which collects gas that has passedthrough containment vessel 30 and over the surface of semiconductorsubstrate 16. If suitable filtering apparatus is available, the used gascan be recycled and sent back to the gas flow source.

The operation of the decontamination device 10 is as follows. First, itis necessary to apply a bias to semiconductor substrate 16 in order tocreate charge accumulation layer 14. In the illustrated embodiment, thesubstrate biasing circuitry 18 creates a positive bias on wafer chuck20, which is insulated by insulating layer 22. The positive bias onwafer chuck 20 causes positive charge accumulation layer 14 to form atthe top of semiconductor substrate 16. This charge accumulation layerappears at the surface as well as below the surface at the top ofsemiconductor substrate 16.

Particle ionizing device 24 then charges contaminating particles 26 withthe same charge sign as charge accumulation layer 14. Here,contaminating particles 26 must be positively charged. Accordingly, thisembodiment uses a source of deep ultraviolet light for a particleionizing device 24. The order of these steps is unimportant for thisembodiment. The particles could be charged before forming the chargeaccumulation layer.

As FIG. 2 illustrates, a deep ultraviolet photon 40 strikescontaminating particle 26, causing the contaminating particle 26 torelease an electron 42. The release of electron 42 makes thecontaminating particle a positively charged ion. Because chargeaccumulation layer 14 also has positive charge, the chargedcontamination particle electrostatically separates from the surface ofsemiconductor substrate 16. Such electrostatic equivalent chargerejection is sufficient to both counter any charge attracting thecontaminating particle to the substrate and to repel the particle fromthe substrate.

FIG. 3 illustrates another embodiment of decontamination device 10 forelectrostatically decontaminating the surface of a semiconductorsubstrate made in accordance with the teachings of the presentinvention. The structure and operation of this embodiment is similar tothe embodiment that FIG. 1 illustrates. Here, however, substrate biasingcircuitry 18 creates a negative bias on wafer chuck 20. The negativebias on wafer chuck 20, in turn, makes charge accumulation layer 14negative.

Because the charge accumulation layer in the embodiment illustrated inFIG. 3 is negative, the particle ionizing device 24 must convert thecontaminating particles 26 into negatively charged ions. Accordingly, inthe embodiment illustrated in FIG. 3, particle ionizing devices 24consist of a source of a low energy, broad area flux of electrons. Inthis embodiment, the contaminating particles should be charged prior toforming the charge accumulation layer because the charge accumulationlayer will deflect the flux of electrons. Alternatively, a method ofnegatively ionizing the particles could be used that does not require anelectron flux, possibly making the order of the steps unimportant. An ACbias could also be applied to semiconductor substrate 16, making theorder of the steps unimportant.

FIG. 4 illustrates more clearly the operation of the embodimentillustrated in FIG. 3. Electron 52 strikes one of the contaminatingparticles 26 on the surface of semiconductor substrate 16. Thecontaminating particle 26 in question absorbs electron 52, thus forminga negatively charged particle, or anion. After anions have been formed,substrate biasing circuitry 18 creates a negative charge accumulationlayer at the top of semiconductor substrate 16. Because chargeaccumulation layer 14 has a negative charge, the negatively chargedcontaminating particle electrostatically repels from the surface ofsemiconductor substrate 16.

An advantage of the embodiments illustrated in FIGS. 1 and 3 is thatthey do not use expensive electronics grade liquids. These embodimentsallow easy removal of submicron-sized particles without the risk offurther contamination. In addition, the disclosed technique allowsremoval of particles smaller than could be removed using currentlyavailable particle removal techniques.

The embodiments of the decontamination devices illustrated in FIGS. 1and 3 use a DC bias on the substrate biasing device to set up the chargeaccumulation layer at the top of the semiconductor substrate. Wheresemiconductors include multiple layers, an AC voltage could be used asan alternative to cause both positive and negative charge accumulationto occur at the semiconductor surface. The disclosed decontaminationdevice also has use as one module in a cluster tool.

FIG. 5 illustrates contaminating particle isolator 44 for collectingcontaminating particles from a semiconductor substrate. The FIG. 5embodiment has a structure similar to decontamination device 10illustrated in FIG. 1. The contaminating particle isolator, however,includes particle collector 46 that collects the charged contaminatingparticles which have been electrostatically repelled from surface ofsemiconductor substrate 16. Particle collector 46 consists of particlecollector bias circuitry 48 that connects to electrode 50. Electrode 50can be positioned in a number of locations inside containment vessel 30of the contaminating particle isolator 44 or inside particle collectionvessel 36.

Particle collector bias circuitry 48 may be constructed of any type ofcircuitry operable to create a charge bias on electrode 50. Electrode 50consists of any material capable of maintaining a charge accumulation.For example, electrode 50 could consist of a piece of semiconductormaterial.

In the FIG. 5 embodiment, particle collector bias circuitry 48 induces anegative charge accumulation on electrode 50. Charged contaminatingparticles repel from the surface of semiconductor substrate 16 as FIGS.1 and 2 describe above. Positively charged particles that have beenelectrostatically repelled from the surface of semiconductor substrate16 can now be collected through the use of electrostatic attraction onthe surface of electrode 50.

After the particles have been collected on the surface of electrode 50,they may be characterized using any known method of characterization.Some currently used methods include time-of-flight secondary ion massspectrometry (TOFSIMS) and x-ray photoelectron spectrometry (XPS).

An advantage of the contaminating particle isolator of the presentembodiment is that many submicron-sized contaminating particles can becollected on electrode 50. Because a large number of particles can becollected in one place, existing characterization techniques can beused. The size of the probe used for the characterization techniquebecomes much less important. In existing characterization techniques,the probe is applied to the substrate surface. Because the surfaceincludes many particles larger than the submicron sized particles, theprobe usually will not identify the submicron-sized particles. Ratherthan attempting to characterize a submicron particle many times smallerthan surrounding particles that will also be detected by the probe, thecurrent invention allows the probe to characterize a large quantity ofcontaminating particles. Presumably, many of the submicron-sizedcontaminating particles will have similar chemical composition. Thisallows existing characterization techniques to identify a contaminantwhere none could be identified before.

The embodiment of contaminating particle isolator 44 that FIG. 6 showshas structure similar to the decontamination device 10 of FIG. 3. Inaddition, particle collector 46 and its components are similar to thecontaminating particle isolator that FIG. 5 shows.

In FIG. 6, particle collector bias circuitry 48 induces a positivecharge accumulation on electrode 50. Negatively charged contaminatingparticles repel from the surface of semiconductor substrate 16 as FIGS.3 and 4 describe above. Charged particles that have beenelectrostatically repelled from the surface of semiconductor substrate16 can now be collected through the use of electrostatic attraction onthe surface of electrode 50. These collected particles can also becharacterized by methods such as TOFSIMS or XPS.

This detailed description of the invention has thus disclosed a methodfor electrostatically decontaminating the surface of a semiconductorsubstrate. The contaminating particles on the semiconductor substrateare ionized to produce ionized particles. The surface of thesemiconductor substrate is biased to produce a charge accumulation layerat the top of the substrate having the same charge sign as the ionizedparticles. Accordingly, contaminated particles become electrostaticallyseparated from the semiconductor surface.

In addition, a method for analytically characterizing particlescontaminating a semiconductor substrate has been described. Inaccordance with this method, contaminating particles on thesemiconductor substrate are ionized to produce ionized particles. Thesemiconductor substrate is biased to produce a charge accumulation layerat the top of the substrate having the same charge sign as the ionizedparticles. These particles then become electrostatically separated fromthe semiconductor surface. The method then employs a charged particlecollector having a charge sign opposite to that of the ejected particlesin order to electrostatically collect the ionized particles. Thechemical composition of the collected ionized particles may then bedetermined.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A decontamination device for electrostaticallydecontaminating the surface of a semiconductor substrate havingcontaminating particles thereon comprising:a substrate biasing deviceoperable to create a charge accumulation layer at the top of thesemiconductor substrate; a particle ionizing device operable to chargethe contaminating particles thereby forming ionized particles having thesame charge sign as the charge accumulation layer.
 2. Thedecontamination device of claim 1 wherein said substrate biasing devicecomprises a wafer chuck.
 3. The decontamination device of claim 1wherein said particle ionizing device comprises a source of deepultraviolet light.
 4. The decontamination device of claim 1 and furthercomprising a gas flow source operable to supply a moving flow of gasinert to the semiconductor substrate over the surface of thesemiconductor substrate.
 5. The decontamination device of claim 1wherein said particle ionizing device comprises a source of a lowenergy, broad area flux of electrons.
 6. The decontamination device ofclaim 1 wherein said substrate biasing device comprises a wafer chuckand said particle ionizing device comprises a source of deep ultravioletlight.
 7. The decontamination device of claim 1 wherein said substratebiasing device comprises a wafer chuck and said particle ionizing devicecomprises a source of a low energy, broad area flux of electrons.
 8. Acontaminating particle isolator for collecting contaminating particlesfrom a semiconductor substrate comprising:a substrate biasing deviceoperable to create a charge accumulation layer at the top of thesemiconductor substrate; a particle ionizing device operable to chargethe contaminating particles, thereby forming ionized particles havingthe same charge sign as the charge accumulation layer; and a particlecollector operable to collect the ionized particles.
 9. Thecontaminating particle isolator of claim 8 wherein said particlecollector comprises a charged electrode containing a charge with acharge sign opposite to that of the ionized particles.
 10. Thecontaminating particle isolator of claim 8 and further comprising:a gasflow source operable to supply a moving flow of a gas inert to thesemiconductor over the surface of the semiconductor substrate.
 11. Thecontaminating particle isolator of claim 8 wherein said substratebiasing device comprises an insulated wafer chuck;said particle ionizingdevice comprises a source of deep ultraviolet light; and said particlecollector comprises a charged electrode containing a charge with acharge sign opposite to that of the ionized particles.
 12. Thecontaminating particle isolator of claim 8 wherein said substratebiasing device comprises an insulated wafer chuck;said particle ionizingdevice comprises a source of a low energy, broad area flux of electrons;and said particle collector comprises a charged electrode containing acharge with a charge sign opposite to that of the ionized particles.